In general, a combined cycle power generation system includes a gas turbine, a steam turbine, a steam cycle, and a multiple pressure combined cycle heat recovery steam generator (HRSG). Steam supply to the steam turbine from the HRSG must be temperature controlled in order to keep the steam temperature from exceeding the rated temperature of the HRSG, interconnecting steam piping, and the steam turbine. One method for controlling and limiting steam temperature includes injection of a water spray into a conventional spray attemperator located upstream (in relation to steam flow) of the final reheater and superheater passes. The water source is typically a high pressure feedwater pump located upstream of at least one economizer disposed within the HRSG.
Though water spray attemperation effectively controls and limits steam temperature, the water used in the spray can contain contaminants damaging to the gas/steam turbine. Water spray attemperation also causes significant reduction in combined cycle performance and efficiency due to the latent heat required for vaporization of the attemperation spray water, which effectively comes from the high level exhaust energy. There is a desire, therefore, for a combined cycle power generation system that controls/limits steam temperature without incurring a performance penalty resulting from use of high level exhaust energy to supply the latent heat of vaporization associated with water spray attemperation.